I agree with a bunch of what you're saying here. That's probably worth saying up front, because I'm going to disagree with a bunch of it, too.
to assume good faith.
"Assume good faith", and nearby concepts such as "be civil", have in the long term severely damaged the social infrastructure of the projects.
I believe that with a strong inner peace conflicts would be less, the atmosphere would improve, and the so-called "editor decline" would be a problem of the past.
I agree that the editor decline is due partly to toxic social atmosphere (mostly, in my experience, on Wikipedia; the smaller sisters are homier, though of course it's hard to know how much of that is simply because they're smaller). Hopefully it's clear that there no way to externally enforce inner peace, and I suggest that attempting to do so is a major source of social toxicity.
That goodwill can be cultivated at upper levels too.
Maybe. Not through "assume good faith", though, which merely gives upper levels an excuse to ignore things they don't want to hear.
Sometimes there are decisions that must be taken to improve our sites, and some of them have
created a lot of drama which maybe could have been minimized by enabling
expression spaces, where there can be some real communication happening, that is, bidirectional, and not to force any ideas, just to foster understanding.
That's a key mistake of reasoning, right there. The Foundation making bad decisions is a problem because the decisions are bad, and any resentment is a secondary concern. The Foundation cannot help making, statistically, bad decisions. The Foundation is intrinsically less qualified than the contributors to make decisions about what direction is in the best interests of the sisterhood; the Foundation's unilateral judgement cannot help being mostly worse for the sisterhood than the contributors' judgement. Can the contributor base make bad decisions? Sure, but the Foundation is at least as likely to be wrong about when that's going to happen as they are likely to be wrong about anything else.
That is, the problem isn't that the Foundation needs to find a better way to liase with the contributors about situations where the Foundation must make unilateral strategic decisions, the problem is that the Foundation is under the delusion there are situations like that. The problem can't be addressed by helping the Foundation to make better unilateral decisions, because it's not possible for the Foundation to be enabled to do so. The problem can't be addressed by improving Foundation-contributor relations, because that doesn't make the Foundations' decisions less damaging to the infrastructure.
In the wikimedia movement there is a serious lack of said expression spaces. For instance, during the WMCON 15, it was the first time that user groups representatives seated down together, also with some WMF employees, to discuss user groups in an open manner. I think it is a big step forward which paves the way in other areas too.
Communicating is much better than not communicating. The Foundation institutionally recognizing its limitations is badly needed.
There is for instance the need to create roads for users to progress in the movement, to bring users from "casual reader" to a "wise wikimedian" status.
Now, that is very true. One really good thing to do for that would be to consistently emphasize users working directly with wiki markup (which teaches by exaemple, something systematically prevented by WYSIWYG), and consistently emphasize everything being done in wiki markup rather than separate languages such as Lua or JavaScript (or, FSM help us all, PHP).
Such a wise people already exist in our movement, it is a pity that we don't enable more knowledge transfer between the "elders" and newcomers, because when one of our wise wikimedian (digitally) dies, it leaves behind a big gap which is very big to fill up again.
Capturing contributor experience, enabling it to be applied and transferred to newcomers, is what I mean to accomplish with my dialog tools https://en.wikinews.org/wiki/Help:Dialog. I mean to transform wiki markup into a medium that can make wikis crowdsourced repositories of know-how about wikis for contributors as well as of knowledge for readers.